No need to make holes in the tank other than for the two small wires. You assemble the micro switch with bobber through the fuel tank fill hole. You don't even need a string on the bobber. Just clip the bobber to the switch lever and glue it in place.
You could also attach a small potentiometer inside the tank and use a float (small bobber) to get analog measurements. Again, all assembly can be make from the tank fill hole.